This invention relates to an automatic level controlling (ALC) circuit for controlling an output signal to a stable level when the level of an input signal such as an audio signal is high.
Generally, sound in the external environment includes a faint sound and loud sound. In the processing of an audio input signal, since it is necessary to assure a wide dynamic range, when the level of the audio input signal is high, it was controlled to a stable level by an automatic level control circuit. In a digital video camera which has spread in recent years, making a low voltage operation has been attempted for reduction of power consumption. It has been demanded for the characteristic of the automatic level controlling circuit to be improved to assure a wide dynamic range.
FIG. 4 is a circuit diagram of a conventional automatic level controlling circuit. In FIG. 4, reference numeral 10 denotes an input terminal to which an audio input signal is supplied; 11 an amplifying circuit for amplifying the input signal; 12 a full-wave rectifying circuit for full-wave rectifying an output from the amplifying circuit; 13 a buffer amplifier; and 14 a detecting terminal to which a capacitor 15 is externally attached. The full-wave rectifying circuit 12 and the capacitor 15 connected to the detecting terminal 14 constitute a detecting circuit. Reference numeral 16 denotes a comparing circuit for comparing a detected voltage Vo appearing at the detecting terminal 14 and a reference voltage Vref from a DC power source. The comparing circuit 16 produces a controlling voltage for controlling the gain of the amplifying circuit 11 according to a comparison result, thereby controlling the level of the output signal therefrom.
FIG. 5 is an input/output characteristic graph of the automatic level controlling circuit having the configuration described above. As seen from FIG. 5, in the absence of the automatic level controlling circuit, the output signal exceeds a saturated level Vs when the level of the input signal exceeds V1, and hence is clipped. On the other hand, in the presence of the automatic level controlling circuit, it restrains the level of the output signal when the level of the input signal reaches V2 so that clipping of the output signal can be prevented.
The characteristic of the automatic level controlling circuit is characterized by not only the above input/output characteristic but also the transient response characteristic when the level of the input signal has changed abruptly, i.e. attack/recovery characteristic.
FIGS. 6A and 6B are waveform charts for explaining the attack characteristic. As seen from FIG. 6A, when the level of the input signal rises abruptly at a time t1, it is amplified instantaneously with a full gain. Then, as seen from FIG. 6B, the level of the output signal is clipped. However, the level is attenuated gradually through the automatic level control operation. In this case, the time taken to reach the stable level, or the time taken until the clipping of the level of the output signal is canceled is called xe2x80x9cattack time TAxe2x80x9d. The attack time is controlled by a CR time constant of the detecting circuit.
FIGS. 7A and 7B are waveform charts for explaining the recovery characteristic. As seen from FIG. 7A, it is assumed that the level of the input signal has decreased abruptly at time t3. Then, it takes a time for the level of the output signal to restore to the ordinary recovery time (t4). This time is called xe2x80x9crecovery time TRxe2x80x9d. In this case, a current flows through a discharging current path (not shown) so that the detected voltage Vo at the detecting terminal decreases.
Where the sensitivity of the automatic level control circuit is high (attack/recovery time is short), the distortion of the output signal increases. On the other hand, where the sensitivity is low (attack/recovery time is long), the output signal disappears. In this way, the performance of the automatic level control circuit is defined by various attack/recovery characteristic.
FIG. 8 is a graph showing the relationship between the detected voltage Vo and the attenuating quantity from the output from the amplifying circuit 11, i.e. an example of the attenuating characteristic curve. This attenuating characteristic curve can be acquired in such a manner that with the input signal fixed to a certain level, the detected voltage Vo is forcibly changed so that the attenuating quantity of the output from the amplifying circuit 11 is measured. The attenuating characteristic curve shown in FIG. 8 exhibits a characteristic attenuating abruptly when the detected voltage increases, in other words, a characteristic suppressing the output signal strongly. However, the automatic level control circuit having such an attenuating characteristic curve gave rise to the following problem.
Generally, as seen from FIGS. 9A and 9B, the recovery characteristic is defined by the recovery time TR1 when the input signal is switched from a middle level signal into a low level signal (reference input signal). FIG. 9A shows the input signal and FIG. 9B shows the response of the output signal (recovery characteristic No. 1).
However, if the input signal is adapted to such a recovery characteristic, the recovery time TR2 when the input signal is switched from a high level signal into the middle level signal becomes very short. FIGS. 10A and 10B show such a recovery characteristic. Specifically, FIG. 10A shows the input signal and FIG. 10B shows the response of the output signal (recovery characteristic No. 2). Therefore, when the input signal is switched from the high level signal into the middle level signal, signal distortion occurs. Particularly, in the case of an audio signal, an uncomfortable phenomenon of xe2x80x9csound breakagexe2x80x9d occurs.
Further, as seen from FIG. 11, the automatic level controlling circuit has a prescribed cover range for an input signal. Specifically, if the input signal is within the cover range, the level of the output signal is limited to a prescribed level. According to the kind of the input signal, it may exceed drastically from the cover range. In this case, it is inevitable that the output signal generates distortion.
However, thereafter, when the level of the input signal becomes low, the time taken for the output signal to restore to a normal stable level, i.e. recovery time will become very long. This problems can be analyzed in detail as follows. As seen from FIG. 11, the detected voltage Vo raises as the level of the input signal rises, thereby limiting the level of the output signal. FIG. 12 is a graph showing a relationship between the detected voltage Vo and an attenuating amount in the output from the amplifier 11. The attenuating amount can be measured by changing the detected voltage Vo with the input signal fixed at a prescribed level. In such an attenuating characteristic, when the detected voltage reaches a saturated region (where the attenuating amount does not vary even when the detected voltage increases), the level of the output signal cannot be limited to a prescribed level and the detected voltage Vo rises abnormally. This greatly prolongs the recovery time.
This invention has been accomplished in view of the inconvenience of the prior art described above. This invention intends to control appropriately the recovery characteristic when an input signal is switched from a high level signal into a middle level signal, thereby minimizing signal distortion.
Further, this invention intends to prevent an abnormal rise in the detected voltage Vo even when the level of the input signal exceeds the cover range, so that the recovery time is shortened to produce an normal output signal.
According to the present invention, the automatic level controlling circuit comprises: an amplifying circuit for amplifying an input signal to produce an output signal therefrom; a detecting circuit for detecting the output signal from said amplifying circuit to produce a detected voltage in a DC level; a comparing circuit for comparing the detected voltage with a first reference voltage to produce a control voltage so that an output level from said amplifying circuit is regulated on the basis of said control voltage, wherein it further comprises a limiter circuit for limiting the control voltage from said comparing circuit so that an attenuating characteristic curve of said amplifying circuit relative to said detected voltage has an inflection point.
In such a configuration of the automatic level control circuit, even when the recovery time is set at the case where the input signal has been switched from the middle level signal into the low level signal (reference input signal), the recovery time when the input signal has been switched from the high level signal into the middle level signal can be prolonged. This permits the signal distortion to be minimized and the xe2x80x9csound breakagexe2x80x9d to be relaxed in the case of the audio signal.
Further, according to the present invention, the automatic level controlling circuit comprises: an amplifying circuit for amplifying an input signal to produce an output signal therefrom; a detecting circuit for detecting the output signal from said amplifying circuit to produce a detected voltage in a DC level; a comparing circuit for comparing the detected voltage with a first reference voltage to produce a control voltage so that an output level from said amplifying circuit is regulated on the basis of said control voltage, wherein it further comprises a limiter circuit for fixing said detected voltage at a prescribed DC voltage when an input signal exceeding a cover range of automatic level control is received.
In such a configuration, in the presence of the limiter circuit, even when the level of the input signal exceeds the cover range, an abnormal rise in the detected voltage can be prevented, thereby shortening the recovery time.
Further, the limiter circuit includes a first transistor with a base to which a DC voltage created by a resistance bleeder is applied and with an emitter which is grounded, and a second transistor with a base to which the emitter of the first transistor is connected and with a collector which is grounded. The emitter of the second transistor is connected to the output from the detected circuit. In this configuration, the detected voltage can be fixed at a stable d.c. voltage with a compensated temperature characteristic.